Related fields include light-emitting diodes (LEDs) and other optoelectronic devices based on III-V materials, and transparent conductive films for optoelectronic devices.
A typical LED emits light from an active photoemissive semiconductor layer sandwiched between p-type and n-type semiconductor layers. Electroluminescence results when negative charge carriers (electrons) from the n-type layer and positive charge carriers (“holes”) from the p-type layer meet and combine in the active photoemissive layer.
The wavelength and color of the emitted light depends, at least in part, on the semiconductor bandgap. For example, arsenides of aluminum (Al), gallium (Ga), indium (In), and their alloys emit red and infrared light; phosphides of Al, Ga, In, and their alloys emit green, yellow, or red light; and nitrides of Al, Ga, In, and their alloys emit ultraviolet, violet, blue, or green light. These “III-V materials,” so-called because they include elements from old group III (now group 13) and old group V (now group 15) of the periodic table, have high carrier mobility and direct bandgaps that are desirable in optoelectronic applications. However, substrates made of III-V materials have historically been very expensive. GaN and AlN substrates are becoming increasingly available, but problems with instability and defects persist. A common alternative approach has been to grow III-V layers by epitaxy on a different substrate material such as sapphire (Al2O3), silicon (Si), silicon carbide (SiC), germanium (Ge), zinc oxide (ZnO), and glass.
A “junction-up” LED emits its output light in a direction pointing away from the substrate, while an inverted, or “flip-chip,” LED emits its output light toward the substrate. Both types may use transparent electrodes on the light-emitting side to facilitate the passage of both electrical current and light through the semiconductor stack. Other devices with similar requirements for current and light traversing the same surface also use transparent electrodes. Many thin-film materials for transparent electrodes are oxides, and are generically known as “transparent conductive oxides” (TCO).
Indium tin oxide, (ITO), the most common TCO material for transparent electrodes, is expensive because it is typically over 90% indium. The optical transparency and the conductivity generally need to be traded off against each other because the highest-transparency formulations are generally different from the highest-conductivity formulations. In addition, both the optical transparency and the conductivity may be unstable with temperature, and therefore may change unpredictably during high-temperature process steps, such as annealing, that may be required to fabricate either the TCO itself or other parts of the device.
As with any other electrode material, another variable that needs to be optimized is the work function. If a low-loss ohmic contact to a semiconductor is desired, a generally necessary (though sometimes insufficient) condition is a close match between the electrode work function and the semiconductor work function. Because p-type and n-type semiconductors have different work functions, so must their electrodes. In some instances this requires the p-type and n-type electrodes to be made from different materials, which is inconvenient and adds to the cost of production.
Therefore, a need exists for a cost-effective TCO material with transmissivity and conductivity that are stable throughout the temperature range of fabrication processes for LEDs and other optoelectronic devices. Preferably, the TCO material should be tunable to match the work functions of both p-type and n-type III-V materials, such as p-GaN and n-GaN (4.2 eV).